Optical scanning apparatus and image-forming apparatus using it

ABSTRACT

Provided are an optical scanning apparatus capable of controlling a focus movement amount to a small value on a surface to be scanned, even with change in the oscillation wavelength of light from a light source and/or with environmental change, and an image-forming apparatus using it. The optical scanning apparatus has a light source, a first imaging optical system for converging light emitted from the light source, a deflector for deflecting the light from the first imaging optical system, a second imaging optical system for scanning the surface to be scanned, with the light deflected by the deflector, and at least one diffraction optical element in the first imaging optical system or in the second imaging optical system. In the optical scanning apparatus, the power of the diffraction optical element is properly set to reduce focus movement on the surface to be scanned, with change in the oscillation wavelength of the light from the light source and focus movement on the surface to be scanned, with change in ambient temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning apparatus and animage-forming apparatus using it and to the optical scanning apparatussuitably applicable to the image-forming apparatus, for example, such aslaser beam printers, digital copiers, etc. involving theelectrophotographic process, which is constructed to converge lightemitted from a light source means, by a first imaging optical system,reflectively deflect the converged light by a polygon mirror as adeflecting means, and optically scan a surface to be scanned, through asecond imaging optical system to record image information thereon. Moreparticularly, the invention relates to the optical scanning apparatuscapable of controlling focus movement amounts to small values on thesurface to be scanned, even with variation in ambient temperature and/orwith change in the initial operating wavelength of the light from thelight source means, so as to be able to keep variation little in thespot size of the beam, thereby always providing good images, and to theimage-forming apparatus using it.

2. Related Background Art

FIG. 5 is a schematic diagram to show the principal part of a lightscanning optical system in a conventional optical scanning apparatus.

In the conventional optical scanning apparatus illustrated in FIG. 5,the light modulated based on image information and emitted from thelight source means 1 is incident to a first imaging optical system L₁comprised of a collimator lens 2, a stop 3, and a cylindrical lens 4.

In this apparatus the light from the light source means 1 is convertedinto nearly parallel light by the collimator lens 2, the nearly parallellight is limited by the aperture stop 3, and the light is then incidentto the cylindrical lens 4 having a predetermined refractive power onlyin the sub-scanning section.

The nearly parallel light incident to the cylindrical lens 4 emerges inthe nearly parallel light state in the main scanning section as it is.

The light is converged in the sub-scanning section to be focused as analmost linear image on a deflective reflection facet 5 a of a polygonmirror 5 (deflecting means). The light reflectively deflected by thepolygon mirror 5 is guided through a second imaging optical system L₂with the fθ characteristic onto a surface to be scanned 7 (a surface ofa photosensitive drum), and the polygon mirror 5 is rotated to opticallyscan the surface to be scanned 7 (the surface of the photosensitivedrum) to record the image information.

For example, Japanese Patent Application Laid-Open No. H06-118346suggests a modification of the optical scanning system of the structureillustrated in FIG. 5, in which focus movement on the surface to bescanned 7 is corrected by applying a diffraction optical element to partof the optics.

In the above application the focal length of a resin condenser lens isset to a value capable of canceling out change in the focal length of aFresnel lens due to variation in the oscillation wavelength of the laserdiode and change in the focal length of the Fresnel lens due tovariation in temperature.

Japanese Patent Application Laid-Open No. H10-333070 describes that thescanning optical apparatus has a first optical system for guiding thelight emitted from the light source means, to the deflecting means and asecond optical system for focusing the light deflected by the deflectingmeans, on the surface to be scanned and a diffraction optical element isprovided in part of at least one optical system of the first and secondoptical systems whereby aberration variation in the sub-scanningdirection of the scanning optical system due to environmental variation(temperature variation) is corrected by change in power of thediffraction optical element and variation in the wavelength of the lightsource means.

In the both cases, the focus movement caused by the optical elementsother than the diffraction optical element is canceled by the powerchange of the diffraction optical element, making use of the propertythat the diffraction optical element greatly changes the power,depending upon the operating wavelength.

Incidentally, factors causing the focus movement on the surface to bescanned include, for example, dispersion or variation of the oscillationwavelength of the light from the light source means and change inambient temperature, but mechanisms of focus variation are greatlydifferent from each other.

The former is nothing but the focus variation affected only by the powerchange of the optical systems due to the variation in the operatingwavelength, whereas the latter is the focus variation caused bycombination of refractive index change of materials and positionaldeviation of the optical elements, change in the oscillation wavelengthof the light from the light source means, etc. due to the change in theambient temperature.

Since these hold independently, the aforementioned suggestions were notalways satisfactory to these factors.

Specifically, the light scanning optical system in Japanese PatentApplication Laid-Open No. 06-118346 failed to include consideration tothe focus movement caused by the environmental change. For example, itis focus movement due to positional variation of the collimator lens andfocus movement caused by change in refractive powers of the collimatorlens, cylindrical lens, fθ lens, etc. due to the change in theoscillation wavelength of the light from the light source means. Forthat reason, the optical system was one that was not always able to makegood correction for focus movement amounts with change of ambienttemperature.

Since the operating wavelength largely varies depending upon the lightsource means under practical use, focus movement amounts caused by thechange of the operating wavelength are unignorable in the light scanningoptical systems using the diffraction optical element that largelychanges the power of the optical system with change in the operatingwavelength. However, the above application failed to take it intoconsideration.

Japanese Patent Application Laid-Open No. H10-333070 gave considerationto the focus movement caused by the change of ambient temperature,including the above factors of focus movement, but failed to take thevariation in the operating wavelength (initial wavelength) of the lightsource means into consideration.

The present invention has been accomplished in order to solve the aboveproblems and an object of the invention is thus to provide an opticalscanning apparatus capable of always forming good images whilecontrolling variation to small values in the spot size of the beam onthe surface to be scanned, by reducing the focus movement amounts evenon the occasion of simultaneous occurrence of the change in theoscillation wavelength of the light from the light source means due todispersion of the oscillation wavelength of the light from the lightsource means and/or due to the change of the ambient temperature, andthe change in refractive indexes of the materials of the optical systemsdue to the change of ambient temperature, and to provide animage-forming apparatus using it.

SUMMARY OF THE INVENTION

An optical scanning apparatus according to one aspect of the inventionis an optical scanning apparatus comprising light source means, a firstimaging optical system for converging light emitted from the lightsource means, deflecting means for deflecting the light from the firstimaging optical system, a second imaging optical system for scanning asurface to be scanned, with the light deflected by the deflecting means,and at least one refraction optical element and one diffraction opticalelement in the first imaging optical system or in the second imagingoptical system,

wherein a power of said diffraction optical element is set to a thirdpower between a first power and a second power, where the first power isa power that the diffraction optical element has when focus movement onthe surface to be scanned, caused by the refraction optical element witha change of an oscillation wavelength of the light from the light sourcemeans, can be canceled by a power change of the diffraction opticalelement and the second power is a power that the diffraction opticalelement has when focus movement on the surface to be scanned, caused bythe refraction optical elements with a change of ambient temperature,can be canceled by a power change of the diffraction optical element.

In the optical scanning apparatus according to another aspect of theinvention, said diffraction optical element has the power in thesub-scanning direction.

In the optical scanning apparatus according to another aspect of theinvention, said diffraction optical element is provided in said firstimaging optical system.

In the optical scanning apparatus according to another aspect of theinvention, said diffraction optical element is placed on a surfaceclosest to said deflecting means in said first imaging optical system.

In the optical scanning apparatus according to another aspect of theinvention, said first imaging optical system comprises a cylindricallens and the diffraction optical element is provided on one surface ofsaid cylindrical lens.

In the optical scanning apparatus according to another aspect of theinvention, where a longitudinal magnification of said second imagingoptical system in the sub-scanning direction is as (times), a focallength fcl (mm) of said cylindrical lens satisfies the followingequation:

fcl≦500/αs.

In the optical scanning apparatus according to another aspect of theinvention, said cylindrical lens includes no position adjusting meansfor adjusting the position in the optical-axis direction.

The optical scanning apparatus according to another aspect of theinvention comprises a third imaging optical system for converging thelight deflected by said deflecting means and guiding the light intolight detecting means,

wherein said first imaging optical system comprises a cylindrical lens,said third imaging optical system comprises an imaging lens having apower at least in the main scanning direction, and said cylindrical lensand said imaging lens are integrally formed.

In the optical scanning apparatus according to another aspect of theinvention, the following equation is satisfied:

|dΔS _(—) T|≧|dΔS_λ| if |dΔST _(—) λ|≧|dΔSλ _(—) T|, or

|dΔS _(—) T|≦|dΔS_λ| if |dΔST _(—) λ|<|dΔSλ _(—) T|,

where dΔSλ_T is a focus movement amount with an ambient temperaturechange when the power of said diffraction optical element is the firstpower; dΔST_λ is a focus movement amount with a change of an initialoperating wavelength of said light source means when the power of saiddiffraction optical element is the second power; dΔS_T is a focusmovement amount with the ambient temperature change and dΔS_λ is a focusmovement amount with the change of the initial operating wavelength ofsaid light source means when the power of said diffraction opticalelement is the third power.

In the optical scanning apparatus according to another aspect of theinvention, the elements are set so that a focus movement amount with achange of 1 nm in the operating wavelength is not more than 0.3 mm.

An image-forming apparatus according to a further aspect of the presentinvention is an image-forming apparatus comprising the scanning opticalapparatus as set forth, a photosensitive body placed on said surface tobe scanned, a developing unit for developing an electrostatic latentimage formed on said photosensitive body with the light under scanningby said scanning optical apparatus, into a toner image, a transfer unitfor transferring said toner image developed, onto a transfer medium, anda fixing unit for fixing the toner image transferred, on the transfermedium.

Another image-forming apparatus according to a further aspect of theinvention is an image-forming apparatus comprising the scanning opticalapparatus as set forth, and a printer controller for converting codedata supplied from an external device, into an image signal andsupplying the image signal to said scanning optical apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are cross-sectional views of principal part inEmbodiment 1 of the present invention;

FIG. 2 is an explanatory diagram to illustrate the principles ofcorrection for focus movement in the sub-scanning direction inEmbodiment 1 of the present invention;

FIG. 3 is a cross-sectional view of principal part in the main scanningdirection in Embodiment 3 of the present invention;

FIG. 4 is a cross-sectional view of principal part in the sub-scanningdirection in Embodiment 5 of the present invention;

FIG. 5 is a schematic diagram of principal part in the conventionallight scanning optical system; and

FIG. 6 is a cross-sectional view of principal part of an image-formingapparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1A is a cross-sectional view of principal part in the main scanningdirection (a main scanning section) of an application wherein theoptical scanning apparatus of Embodiment 1 of the present invention isapplied to an image-forming apparatus such as a laser beam printer, adigital copier, or the like, and FIG. 1B is a cross-sectional view ofprincipal part in the sub-scanning direction (a sub-scanning section) ofa part of FIG. 1A.

In the figures, light emitted from a semiconductor laser 1 being thelight source means travels through a first imaging optical systemconsisting of a collimator lens 2, a stop 3, and a cylindrical lens 4 tobe incident to a deflector means 5. Here the light from thesemiconductor laser 1 is converted into a parallel beam by thecollimator lens 2, the parallel beam is limited by the stop 3 to enterthe cylindrical lens 4, the cylindrical lens 4 transmits the parallellight in the main scanning direction as it is, but converges it in thesub-scanning direction to focus it as a linear image longitudinal in themain scanning direction, on a deflection facet 5 a of the deflectormeans 5.

The deflector means 5 is comprised of a polygon mirror, which is rotatedat an equal speed about the rotation center of the rotation axis 5 b andwhich reflectively deflects the light emitted from the semiconductorlaser 1. The light reflectively deflected by the polygon mirror 5 isconverged in the both main scanning and sub-scanning directions by asecond imaging optical system 6 with the fθ characteristic comprised oftwo fθ lenses 6 a, 6 b, which are a first fθ lens 6 a and a second fθlens 6 b, and optically scans a surface of a photosensitive drum 7 beinga surface to be scanned.

In the present embodiment, the cylindrical lens 4 and the two fθ lenses6 a, 6 b all are toric lenses, which are made of a plastic material. Acylindrical surface with curvature (R) only in the sub-scanningdirection is formed in the semiconductor-laser-1-side surface of thecylindrical lens 4, the polygon-mirror-5-side surface thereof is aplane, and a diffraction grating with power (optical power) only in thesub-scanning direction is formed as a diffraction optical element 8 onthe plane.

Table 1 presents numerical values for the structure of the lightscanning optical system in the present embodiment. The entrance-sidesurface of each lens is denoted by R1 and the exit-side surface of eachlens by R2.

TABLE 1 structure of light scanning optical system in Embodiment 1radius of surface refractive curvature Rs(mm) separation d(mm) index Nsemiconductor laser 23.585 1.00000 collimator lens R1 182.212 2.0001.76203 collimator lens R2 −20.831 13.450 1.00000 cylindrical lens R127.086 3.000 1.49101 (radius of curvature in sagittal direction)cylindrical lens R1 ∞ 35.600 1.00000 (radius of curvature in sagittaldirection) deflection facet of ∞ 10.500 1.00000 polygon mirror first fθlens R1 −10.000 6.500 1.52420 (radius of curvature in sagittaldirection) first fθ lens R2 −22.950 7.120 1.00000 (radius of curvaturein sagittal direction) second fθ lens R1 110.239 6.600 1.52420 (radiusof curvature in sagittal direction) second fθ lens R2 −12.117 103.2801.00000 (radius of curvature in sagittal direction) surface to bescanned ∞

The shape of the diffraction grating formed on the R2 surface of thecylindrical lens 4 is represented by Eq. (1) below and the coefficientsof the phase terms thereof are presented in Table 2, where φ(y,z) is aphase function, the origin is set at an intersection with the opticalaxis, the x-axis is taken along the direction of the optical axis, they-axis along the axis perpendicular to the optical axis in the mainscanning plane, and the z-axis along the axis perpendicular to theoptical axis in the sub-scanning plane.

φ(y,z)=(2π/λ)(c ₁ z ² +c ₂ y ² z ² +c ₃ y ⁴ z ²)  (1)

C₁ to C₃: phase polynomial coefficients, λ=780 nm

TABLE 2 phase terms of diffraction optical element C₁ −5.08566E−3 C₂0.00000 C₃ 0.00000

Next described is the correction for the focus movement on the surfaceto be scanned 7 due to the change in the oscillation wavelength of thesemiconductor laser 1 and due to the change in the ambient temperature.

First described referring to FIG. 2 are the principles of the correctionfor the focus movement in the sub-scanning direction in the presentembodiment. FIG. 2 is a cross-sectional view of the main part whereinthe optical paths in the sub-scanning direction of the light scanningoptical system of the present embodiment are expanded, which shows astate of imaging relation between in a reference state (solid lines) andin a state with correction for focus (dashed lines).

For example, suppose there was an increase of the ambient temperature inthe light scanning optical system. The cylindrical lens 4 and fθ lenses6 a, 6 b are made of plastic, and in general, plastic has the propertyof lowering the refractive index with increase in temperature. For thisreason, the refractive power of the overall system decreases, so thatthe focus position in the sub-scanning direction moves from point P topoint Q in the drawing away from the surface to be scanned 7, so as tocause focus deviation.

However, the semiconductor laser 1 has the property of increasing itsoscillation wavelength with increase in the temperature. The longer theoscillation wavelength, the larger the angles of diffraction at thediffraction optical element 8 become. This causes the linear imageoriginally located at a point R near the deflective reflection facet 5 aof the polygon mirror 5, to move to the position of a point S on thesemiconductor laser 1 side.

Then, the focus position, which was to move to the point Q mainlybecause of the decrease of the refractive index of the cylindrical lens4 and fθ lenses 6 a, 6 b, is returned again to the point P. Namely, thiseffects temperature compensation.

There is also dispersion of the oscillation wavelength in the initialstate among semiconductor lasers 1 and the operating wavelength differsdepending upon the individual semiconductor lasers used.

For example, suppose there was change in the initial wavelength of thesemiconductor laser 1 and the operating wavelength became longer thanthe reference wavelength. The lenses including the collimator lens 2,the cylindrical lens 4, the fθ lenses 6 a, 6 b, and so on have theproperty of lowering the refractive index with increase in the operatingwavelength, and the focus position in the sub-scanning direction movesfrom the point P to the position of the point Q, so as to cause focusdeviation.

However, the longer the operating wavelength, the larger the angles ofdiffraction become at the diffraction optical element 8. This increasesthe power of the diffraction optical element to return the focusposition, which was to move to the point Q as described above, to theposition of the point P. Namely, color compensation is effected.

Based on the principles as described above, the present embodimentachieves well-balanced correction between the focus movement due to thechange in the ambient temperature and the focus movement due to theinitial wavelength change in the oscillation wavelength of the lightsource means occurring independently of each other, by optimally settingthe power of the diffraction optical element 8 in the sub-scanningdirection.

The present embodiment will be described further below using specificnumerical values.

Focus variation will be compared among various types of diffractionoptical elements under the conditions that the focal length (in thesub-scanning section) of the cylindrical lens 4 in the presentembodiment is fcl=35.81 mm, the lens back thereof is Sk_cl=34.50 mm, theinitial wavelength variation of the light source means 1 is ±15 nm, thechange of the ambient temperature is ±25° C.

Under these circumstances, the temperature coefficient of the wavelengthchange of the semiconductor laser as the light source means is 0.255nm/° C., the wavelength change with the ambient temperature change of+25° C. is +6.375 nm, the refractive index change of the cylindricallens 4 and fθ lenses 6 a, 6 b with the ambient temperature change of+25° C. is −0.00198, and the refractive index change with the wavelengthchange of +6.375 nm is −0.00014.

The lens barrel of the collimator lens 2 is made of two types of resin,one of which is noryl (modified PPO) having the coefficient of linearexpansion of 2.3 and the other of which is polysulfone having thecoefficient of linear expansion of 5.6. With the ambient temperaturechange of +25° C., the position of the collimator lens moves 19.4 μmtoward the polygon mirror.

Then let us define focus movement amounts caused by the respectiveoptical elements as follows. With the wavelength change of 6.375 nm dueto the ambient temperature change of the light source means 1, Co1 λ1represents a focus movement amount by the collimator lens 2, CL R λ1 afocus movement amount by the refracting surface of the cylindrical lens4, CL DOE λ1 a focus movement amount by the diffracting surface of thecylindrical lens 4, and fθ λ1 a focus movement amount by the fθ lensunit 6; with the refractive index change due to the ambient temperaturechange of 25° C., CL RN represents a focus movement amount by therefracting surface of the cylindrical lens 4, and fθ N a focus movementamount by the fθ lens unit 6.

With the initial wavelength change of 15 nm of the semiconductor laser1, Co1 λ2 represents a focus movement amount by the collimator lens 2,CL R λ2 a focus movement amount by the refracting surface of thecylindrical lens 4, CL DOE λ2 a focus movement amount by the diffractingsurface of the cylindrical lens 4, and fθ λ2 a focus movement amount bythe fθ lens unit 6.

It is, however, noted that the focus movement amount Co1 λ2 by thecollimator lens 2 with the initial wavelength change of thesemiconductor laser 1 is canceled, because the focus adjustment of thecollimator lens 2 is carried out on the occasion of mounting thesemiconductor laser 1 and the collimator lens 2 into a light sourceunit.

In the present embodiment, a type in which the power of the diffractionoptical element 8 (first power) is set so as not to cause focus movementeven with change of the oscillation wavelength of the light source means1 from the reference wavelength, will be referred to hereinafter as anachromatic type.

The focus movement amounts by the respective components in the case ofthe achromatic type are presented in Table 3 below.

TABLE 3 focus movement of achromatic type with ambient temperature withinitial wavelength change of +25° C. change of +15 nm CL R λ1 (focus0.099 (mm) CL R λ2 0.233 (mm) movement amount by refracting surface ofcylindrical lens) CL R N (focus 1.403 movement amount with refractiveindex change of cylindrical lens) CL DOE λ1 −0.226 CL DOE λ2 −0.531(focus movement amount by diffracting surface of cylindrical lens) Colλ1 (focus 0.170 movement amount by collimator lens) Col d (focus −0.451movement amount with wavelength change of collimator lens) fθ λ1 (focus0.126 fθ λ2 0.298 movement amount by fθ lens 6) fθ N (focus 1.788movement amount with refractive index change of fθ lens 6) focusmovement 2.910 focus movement 0.000 amount with amount with ambientinitial temperature wavelength change of +25° C. change of 15 nm

In the achromatic type the focus movement amount is large with theambient temperature change, so as to cause heavy degradation ofperformance. Further, since the ambient temperature change is thephenomenon that can occur in any optical scanning apparatus, differentfrom the dispersion of the oscillation wavelength of the light sourcemeans, it is desirably set to a relatively small value.

A type in which the power of the diffraction optical element (secondpower) is set so as not to cause the focus movement with the ambienttemperature change and in which the lens back of the cylindrical lens 4is the same as Sk_cl=34.50 mm, will be referred to hereinafter as anoverall temperature compensation type.

The focus movement amounts by the respective components in the case ofthe overall temperature compensation type are presented in Table 4.

TABLE 4 focus movement of overall temperature with ambient temperaturewith initial wavelength change of +25° C. change of +15 nm CL R λ1 0.032(mm) CL R λ2 0.076 (mm) CL R N 0.458 CL DOE λ1 −2.125 CL DOE λ2 −4.999Col λ1 0.170 Col d −0.451 fθ λ1 0.126 fθ λ2 0.298 fθ N 1.788 focusmovement 0.000 focus movement −4.625 amount with amount with ambientinitial temperature wavelength change of +25° C. change of 15 nm

Since in the overall temperature compensation type the focus movementamount due to the dispersion of the initial wavelength of the lightsource means is too large, it takes most of the designed depth, so as toheavily lower non-defective percentage of optical scanning apparatus andthus pose the problem of cost increase.

Therefore, the power of the diffraction optical element is set to a type(in which the power of the diffraction optical element will be referredto as third power) located between the achromatic type and the overalltemperature compensation type to make a balance between the focusmovement amount due to the initial wavelength change of the light fromthe light source means and the focus movement amount due to the ambienttemperature change, thereby decreasing the focus movement amount on thesurface to be scanned.

Namely, this realizes the optical scanning apparatus with less variationin the spot size.

Since in the present embodiment the diffraction grating is formed onlyon the cylindrical lens 4, the type of the power of the diffractionoptical element (either the achromatic type, the overall temperaturecompensation type, or the type between them) can be discriminated byfinding the variation amount of the lens back of the cylindrical lens 4due to the ambient temperature change.

Table 5 shows the relation of focus movement amounts on the surface tobe scanned, with the types of power of the diffraction optical element.

TABLE 5 FOCUS MOVEMENT AMOUNT IN SUB-SCANNING SECTION dΔS(mm)

LENS BACK CHANGE AMOUNT OF CL WITH AMBIENT TEMPERATURE CHANGE OF 25° C.ΔSk_cl (mm)

With the change of ambient temperature of +25° C., the lens backvariation amount of the cylindrical lens is ΔSk_cl=+0.12 mm in theachromatic type and ΔSk_cl=−0.16 in the overall temperature compensationtype. Namely, a balance can be made between the focus movement amountdue to the initial wavelength change of the light source means and thefocus movement amount due to the ambient temperature change by settingthe variation of the lens back of the cylindrical lens with the changeof ambient temperature of +25° C., ΔSk_cl, in the range of +0.12 to−0.16.

In the present embodiment they are balanced by setting the variationamount of the lens back of the cylindrical lens with the change ofambient temperature of +25° C. to ΔSk_cl=0.00.

From computation based on the results of Table 1 and Table 2 inEmbodiment 1, the refractive power φ1 of the R1 surface of thecylindrical lens is 0.01813 and the diffractive power φ2 of the R2surface of the cylindrical lens is 0.01017. Hence the power ratio isφ2/φ1=0.561. With e=d/N, φ=1/f=φ1+φ2−e·φ1·φ2, where f is the focallength, N the refractive index of the cylindrical lens, and d thesurface separation of the cylindrical lens; and Δ=Sk−f where Sk is thelens back.

Table A below also presents the power ratios of the achromatic type andthe overall temperature compensation type as comparative examples.

C2 and C3 of the achromatic type and the overall temperaturecompensation type are the phase polynomial coefficients, which are zeroas in Table 2.

As apparent from Table A below, the present embodiment meets therelation of φ2 of the achromatic type (first power)<φ2 of Embodiment 1(third power)<φ2 of the overall temperature compensation type (secondpower).

TABLE A Comparative Comparative example 2: example 1: overalltemperature achromatic type compensation type Embodiment 1 (first power)(second power) R1 27.086 19.307 59.106 DOE C1 −5.0856 E−03 −1.09122 E−03−1.02685 E−02 d 3.000 3.000 3.000 N 1.491014 1.491014 1.491014 φ10.01813 0.02543 0.00831 φ2 0.01017 0.00218 0.02054 e 2.012 2.012 2.012 φ2.79 E−02 2.75 E−02 2.85 E−02 f 35.806 36.361 35.086 Δ −1.306 −1.861−0.586 Sk 34.500 34.500 34.500 ΔSk 0.00 0.12 −0.16 power ratio 0.5610.086 2.472

Table 6 below presents the focus movement amounts by the respectivecomponents in the light scanning optical system of the type of thediffraction optical element wherein the power of the diffraction opticalelement of the present embodiment is the third power.

TABLE 6 focus movement in Embodiment 1 with ambient temperature withinitial wavelength change of +25° C. change of +15 nm CL R λ1 0.071 (mm)CL R λ2 0.166 (mm) CL R N 1.000 CL DOE λ1 −1.052 CL DOE λ2 −2.476 Col λ10.170 Col d −0.451 fθ λ1 0.126 fθ λ2 0.298 fθ N 1.788 focus movement1.652 focus movement −2.012 amount with amount with ambient initialtemperature wavelength change of +25° C. change of 15 nm

In this case, the focus movement amount due to the initial wavelengthchange of +15 nm of the light source means is dΔS=−2.012 mm and thefocus movement amount due to the ambient temperature change of +25° C.is dΔS=+1.652 mm, thereby making a balance between the two focusmovement amounts. Since the initial wavelength change of the lightsource means and the ambient temperature change both can also appearreverse, the focus movement amount is considered by its absolute value.

In the present embodiment this permits the cylindrical lens 4 to beplaced without adjustment of focus in the direction of the optical axis,thus achieving advantages of reduction of assembling time, decrease oftools, etc., as well as the cost merit.

Embodiment 2

A principal difference of Embodiment 2 from Embodiment 1 is that thetype of power of the diffraction optical element is changed while thelens back of the cylindrical lens 2 is kept at the same as Sk_cl=34.50mm.

The type of power of the diffraction optical element was set so that thelens back variation amount of the cylindrical lens with the change ofambient temperature of +25° C. was ΔSk_cl=+0.02 mm.

Let dΔSλ_T be a focus change amount with change of ambient temperatureof 25° C. in the light scanning optical system wherein the power of thediffraction optical element is set to the achromatic type in Table 3,dΔST_λ be a focus change amount with change of 15 nm in the initialwavelength of the light from the light source means 1 in the lightscanning optical system wherein the power is set to the overalltemperature compensation type in Table 4, dΔS_λ be a focus change amountwith change of 15 nm in the initial wavelength of the light from thelight source means in the light scanning optical system wherein thediffraction optical element is set to a certain type, and dΔS_T be afocus change amount with change of ambient temperature of 25° C. in thelight scanning optical system with the diffraction optical element ofthe certain tape. In the present embodiment, since |dΔSλ_T|<|dΔST_λ|,the apparatus is set so as to satisfy the following relation:

|dΔS _(—) T|≧|dΔS_λ|.

The configuration of the cylindrical lens 4 in the present embodiment ispresented in Table 7 below.

TABLE 7 configuration of cylindrical lens radius of curvature insagittal 25.373 direction of incidence surface phase terms ofdiffraction optical element C₁ −4.42494E−3 C₂ 0.00000 C₃ 0.00000

Table 8 below shows the focus change amounts by the respectivecomponents in the light scanning optical system with the type of thediffraction optical element.

TABLE 8 focus movement in Embodiment 2 with ambient temperature withinitial wavelength change of +25° C. change of +15 nm CL R λ1 0.075 (mm)CL R λ2 0.178 (mm) CL R N 1.068 CL DOE λ1 −0.916 CL DOE λ2 −2.154 Col λ10.170 Col d −0.451 fθ λ1 0.126 fθ λ2 0.298 fθ N 1.788 focus movement1.861 focus movement −1.679 amount with amount with ambient initialtemperature wavelength change of +25° C. change of 15 nm

In the present embodiment, the focus movement amount due to the ambienttemperature change is dΔS_T=1.861 mm, the focus movement amount due tothe initial wavelength change of the light source means isdΔS_(—)λ=1.679 mm, so as to make a balance between the focus movementamounts due to the two factors, and the sum of the focus movementamounts is controlled to the small value of 3.540 mm.

From computation based on the results of Table 7 and Table 8 inEmbodiment 2, the refractive power φ1 of the R1 surface of thecylindrical lens is 0.01935 and the diffractive power φ2 of the R2surface of the cylindrical lens is 0.00885. Hence the power ratio isφ2/φ1=0.457. With e=d/N, φ=1/f=φ1+φ2−e·φ1·φ2, where f is the focallength, N the refractive index of the cylindrical lens, and d thesurface separation of the cylindrical lens; and Δ=Sk−f where Sk is thelens back.

Table B below also presents the power ratios of the achromatic type andthe overall temperature compensation type as comparative examples.

As apparent from Table A and Table B below, the present embodiment meetsthe relation of φ2 of the achromatic type (first power)<φ2 of Embodiment2 (third power)<φ2 of the overall temperature compensation type (secondpower).

TABLE B Embodiment 2 R1 25.373 DOE C1 −4.42494E−03 d 3.000 N 1.491014 φ10.001935 φ2 0.00885 e 2.012 φ 2.79E−02 f 35.897 Δ −1.398 Sk 34.500 ΔSk0.02 power ratio 0.457

This allows us to construct the optical scanning apparatus capable ofalways providing good images while reducing variation in the spot sizeon the surface to be scanned, even with the ambient temperature changeand/or with the initial wavelength change.

Embodiment 3

Main differences of Embodiment 3 from Embodiment 1 are that the lightsource means 1 is one with less dispersion of initial wavelength, ±5 nm,and that the type of power of the diffraction optical element 8 ischanged while keeping the lens back of the cylindrical lens 4 at thesame as Sk_cl=34.50 mm.

The type of power of the diffraction optical element 8 was set so thatthe lens back variation amount of the cylindrical lens 4 with the changeof ambient temperature of +25° C. became ΔSk_cl=−0.10 mm.

The cylindrical lens 4 of the present embodiment is constructed in thestructure presented in Table 9 below.

TABLE 9 configuration of cylindrical lens radius of curvature insagittal 41.369 direction of incidence surface phase terms ofdiffraction optical element C₁ −8.41292E−3 C₂ 0.00000 C₃ 0.00000

Table 10 presents the relation of focus movement amounts on the surfaceto be scanned 7, with the types of power of the diffraction opticalelement 8.

TABLE 10 FOCUS MOVEMENT AMOUNT IN SUB-SCANNING SECTION dΔS(mm)

LENS BACK CHANGE AMOUNT OF CL WITH AMBIENT TEMPERATURE CHANGE OF 25° C.ΔSk_cl (mm)

Under the aforementioned conditions, the focus movement amount isdΔSλ_T=2.910 mm with the change of ambient temperature of 25° C. in thelight scanning optical system wherein the power of the diffractionoptical element 8 is set to the achromatic type (ΔSk_cl=0.12 mm), andthe focus movement amount is dΔST_(—)λ=1.542 mm with the change of 5 nmin the initial wavelength of the light source means 1 in the lightscanning optical system wherein the power of the diffraction opticalelement 8 is set to the overall temperature compensation type(ΔSk_cl=−0.16 mm).

From computation based on the results of Table 9 of Embodiment 3, therefractive power φ1 of the R1 surface of the cylindrical lens is 0.01187and the diffractive power φ2 of the R2 surface of the cylindrical lensis 0.01683. Hence the power ratio is φ2/φ1=1.418. With e=d/N,φ=1/f=φ1+φ2−e·φ1·φ2, where f is the focal length, N the refractive indexof the cylindrical lens, and d the surface separation of the cylindricallens; and Δ=Sk−f where Sk is the lens back.

As apparent from Table A and Table C below, the present embodiment meetsthe relation of φ2 of the achromatic type (first power)<φ2 of Embodiment3 (third power)<φ2 of the overall temperature compensation type (secondpower).

TABLE C Embodiment 3 R1 41.369 DOE C1 −8.41292E−03 d 3.000 N 1.491014 φ10.001187 φ2 0.001683 e 2.012 φ 2.83E−02 f 35.344 Δ −0.844 Sk 34.500 ΔSk−0.10 power ratio 1.418

In the present embodiment, since

|dΔSλ _(—) T|>|dΔST_λ|,

the apparatus is set so as to satisfy the following relation:

|dΔS _(—) T|≦|dΔS_λ|.

Table 11 below presents the focus movement amounts by the respectivecomponents in the light scanning optical system with the type of thediffraction optical element 8 in the present embodiment.

TABLE 11 focus movement in Embodiment 3 with ambient temperature withinitial wavelength change of +25° C. change of +15 nm CL R λ1 0.046 (mm)CL R λ2 0.036 (mm) CL R N 0.655 CL DOE λ1 −1.741 CL DOE λ2 −1.365 Col λ10.170 Col d −0.451 fθ λ1 0.126 fθ λ2 0.099 fθ N 1.788 focus movement0.594 focus movement −1.230 amount with amount with ambient initialtemperature wavelength change of +25° C. change of 15 nm

In the present embodiment, the focus movement amount due to the ambienttemperature change is dΔS_T=0.594 mm and the focus change amount due tothe initial wavelength change of the light source means 1 isdΔS_(—)λ=−1.230 mm, so as to make a balance between the focus movementamounts due to the two factors. In the light scanning optical systemwherein the operating wavelength is λ=775 nm because of change of theinitial wavelength of the light source means 1, Δλ=−5 nm, from thereference wavelength λ₀=780 nm, when the ambient temperature changes byΔT=+25° C. from the ordinary temperature T_(o)=25° C. to T=50° C., thefocus movement due to the initial wavelength change of the light sourcemeans 1 and the focus change due to the ambient temperature change occurin the same direction away from the light source means, and thusvariation of the spot size can be restrained by also decreasing thetotal of the focus movement. The present embodiment implements it.

Further, FIG. 3 is a cross-sectional view in the main scanning directionof the optical scanning apparatus in Embodiment 3.

The light scanning optical system of the present embodiment is providedwith a third imaging optical system 9 for detecting a synchronous signalof scanning, which includes an imaging lens 9 for guiding the lightdeflected by the polygon mirror 5, to a synchronism detection means 10.The imaging lens 9 is made of plastic so as to be integral with thecylindrical lens 4, which can decrease the number of components, therebyrealizing cost reduction of the optical scanning apparatus.

Embodiment 4

A main difference between Embodiment 4 and Embodiment 1 is that the lensback of the cylindrical lens 4 is Sk_cl=20.00 mm.

The type of the diffraction optical element is the same as in Embodiment1 and the change amount of the lens back of the cylindrical lens withthe change of ambient temperature of +25° C. is ΔSk_cl=0.00.

The configuration of the cylindrical lens in the present embodiment ispresented in Table 12 below.

TABLE 12 configuration of cylindrical lens radius of curvature insagittal 16.211 (mm) direction of incidence surface phase terms ofdiffraction optical element C₁ −8.87256E−3 C₂ 0.00000 C₃ 0.00000

From computation based on the results of Table 12 of Embodiment 4, therefractive power φ1 of the R1 surface of the cylindrical lens is 0.03029and the diffractive power φ2 of the R2 surface of the cylindrical lensis 0.01775. Hence the power ratio is φ2/φ1=0.586. With e=d/N,φ=1/f=φ1+φ2−e·φ1·φ2, where f is the focal length, N the refractive indexof the cylindrical lens, and d the surface separation of the cylindricallens; and Δ=Sk−f where Sk is the lens back.

TABLE D Embodiment 4 R1 16.211 DOE C1 −8.87256E−03 d 3.000 N 1.491014 φ10.03029 φ2 0.01775 e 2.012 φ 4.70E−02 f 21.298 Δ −1.298 Sk 20.000 ΔSk−0.00 power ratio 0.586

Table 13 below presents the focus change amounts by the respectivecomponents in the light scanning optical system with the type of thediffraction optical element.

TABLE 13 focus movement in Embodiment 4 with ambient temperature withinitial wavelength change of +25° C. change of +15 nm CL R λ1 0.041 (mm)CL R λ2 0.095 (mm) CL R N 0.574 CL DOE λ1 −0.627 CL DOE λ2 −1.475 Col λ10.056 Col d −0.158 fθ λ1 0.126 fθ λ2 0.298 fθ N 1.788 focus movement1.808 focus movement −1.082 amount with amount with ambient initialtemperature wavelength change of +25° C. change of 15 nm

In the present embodiment, the lens back of the cylindrical lens ischanged from that of Embodiment 1. When this is compared by focallengths, fcl=35.81 mm in Embodiment 1, whereas fcl=21.30 mm in thepresent embodiment.

This permits the focus movement amount due to change of operatingwavelength to be controlled to a small amount by setting the focallength of the cylindrical lens shorter. This is because the movementamount of the focal line position after passage through the cylindricallens due to the wavelength change becomes smaller and because the focusmovement amount calculated by multiplying the movement amount of thefocal line position by the longitudinal magnification αs=11.04 of thesecond imaging lens in the sub-scanning direction, becomes smaller.

When the focal length of the cylindrical lens having the diffractionoptical element is fcl and the longitudinal magnification of the secondimaging optical system 6 in the sub-scanning direction is as, correctioncan be made in the level where no problem occurs in practical use,within the range satisfying Eq. (2) below.

fcl≦500/αs  (2)

Embodiment 5

FIG. 4 is a cross-sectional view in the sub-scanning direction of theoptical scanning apparatus in Embodiment 5.

In the present embodiment, the cylindrical lens 4 and two fθ lenses 6 a,6 b are made of synthetic resin, the semiconductor-laser-1-side surfaceof the cylindrical lens 4 is a cylindrical surface with curvature (R)only in the sub-scanning direction, the polygon-mirror-5-side surface ofthe fθ lens 6 b is a plane, and the diffraction optical element 8 withpower only in the sub-scanning direction is formed on the plane.

The configuration of the light scanning optical system of the presentembodiment is presented in Table 14 and the shape of the diffractionoptical element formed on the incidence surface of the second fθ lens 6b in Table 15.

TABLE 14 structure of light scanning optical system in Embodiment 5radius of surface curvature separation refractive Rs (mm) d (mm) index Nsemiconductor laser 23.585 1.00000 collimator lens R1 182.212 2.0001.76203 collimator lens R2 −20.831 13.450 1.00000 cylindrical lens R1−19.117 3.000 1.52420 (radius of curvature in sagittal direction)cylindrical lens R1 ∞ 35.600 1.00000 (radius of curvature in sagittaldirection) deflection facet of ∞ 10.500 1.00000 polygon mirror first fθlens R1 −10.000 6.500 1.52420 (radius of curvature in sagittaldirection) first fθ lens R2 −22.950 7.120 1.00000 (radius of curvaturein sagittal direction) second fθ lens R1 ∞ 6.600 1.52420 (radius ofcurvature in sagittal direction) second fθ lens R2 −14.300 103.2801.00000 (radius of curvature in sagittal direction) surface to bescanned ∞

TABLE 15 phase terms of diffraction optical element C₁ −6.75000E−3 C₂0.00000 C₃ 0.00000

In the present embodiment the ambient temperature change and the initialwavelength change of the light source means are similar to those inEmbodiment 1. In the present embodiment the diffraction optical element8 is formed on one surface in the second imaging means and the type ofpower of the diffraction optical element 8 can be defined by thedirection of focus movement.

In the present embodiment, the power of the diffraction optical element8 is determined so that correction is insufficient for the focusmovement due to the ambient temperature change and correction is overfor the focus movement due to the initial wavelength change of the lightsource means.

Namely, the power is set between the achromatic type and the overalltemperature compensation type, as in Embodiment 1.

In the light scanning optical system with the type of the diffractionoptical element in the present embodiment, where fθ DOE λ represents thefocus movement at the diffracting surface of the fθ lens with theinitial wavelength change of the light source means, the focus movementamounts by the respective components are as follows.

TABLE 16 focus movement in Embodiment 5 with ambient temperature withinitial wavelength change of +25° C. change of +15 nm CL R λ1 0.114 (mm)CL R λ2 0.267 (mm) CL R N 1.608 Col λ1 0.189 Col d −0.501 fθ λ1 0.094 fθλ2 0.221 fθ N 1.381 fθ DOE λ1 −0.930 fθ DOE λ2 −2.189 focus movement1.905 focus movement −1.700 amount with amount with ambient initialtemperature wavelength change of +25° C. change of 15 nm

In the present embodiment, the focus movement amount with the change ofambient temperature of 25° C. is dΔS_T=1.905 mm and the focus movementamount with the change of 15 nm in the initial wavelength of the lightsource means is dΔS_(—)λ=1.700 mm, whereby well-balanced correction canbe made for the two focus movement amounts even in the case wherein thediffraction optical element 8 is placed in the second imaging opticalsystem.

Degrees of design freedom of the fθ lens can be increased by placing thediffraction optical element on the fθ lens as in the present embodiment.

Concerning the optical scanning apparatus in the respective embodiments,the above described the methods of effecting the balanced correction forthe focus movement in the sub-scanning direction due to the ambienttemperature change and due to the initial wavelength change of the lightsource means by using one diffraction optical element in the first orsecond imaging optical system and setting the power thereof to theoptimum value, but the diffraction optical element does not have to belimited to one; for example, two diffraction optical elements can beprovided on the cylindrical lens in the first imaging optical system andon the fθ lens in the second imaging optical system, or threediffraction optical elements can be provided on the collimator lens inthe first imaging optical system and on the first fθ lens and second fθlens in the second imaging optical system.

The focus movement in the sub-scanning direction was described in theembodiments of the present invention, but it should be noted that thefocus movement in the main scanning direction can also be correctedreadily, of course, by using the diffraction optical element having thediffraction action in the main scanning direction.

Described next is an image-forming apparatus applied to the presentinvention.

FIG. 6 is a cross-sectional view of the principal part along thesub-scanning direction to show an embodiment of the image-formingapparatus of the present invention. In FIG. 6, numeral 104 designatesthe image-forming apparatus. This image-forming apparatus 104 acceptsinput of code data Dc from an external device 117 such as a personalcomputer or the like. This code data Dc is converted into image data(dot data) Di by a printer controller 111 in the apparatus. This imagedata Di is supplied to an optical scanning unit 100 having the structureas described in either of Embodiments 1 to 5. This optical scanning unit100 outputs an optical beam 103 modulated according to the image data Diand this light beam 103 scans a photosensitive surface of photosensitivedrum 101 in the main scanning direction.

The photosensitive drum 101 as an electrostatic latent image carrier(photosensitive body) is rotated clockwise by a motor 115. With therotation thereof, the photosensitive surface of the photosensitive drum101 moves in the sub-scanning direction perpendicular to the mainscanning direction, relative to the light beam 103. Above thephotosensitive drum 101, a charging roller 102 for uniformly chargingthe surface of the photosensitive drum 101 is disposed so as to contactthe surface. Then the surface of the photosensitive drum 101 charged bythe charging roller 102 is exposed to the light beam 103 under scanningby the optical scanning unit 100.

As described previously, the light beam 103 is modulated based on theimage data Di and an electrostatic latent image is formed on the surfaceof the photosensitive drum 101 under irradiation with this light beam103. This electrostatic latent image is developed into a toner image bya developing unit 107 disposed so as to contact the photosensitive drum101 downstream in the rotating direction of the photosensitive drum 101from the irradiation position of the light beam 101.

The toner image developed by the developing unit 107 is transferred ontoa sheet 112 being a transfer medium, by a transfer roller 108 opposed tothe photosensitive drum 101 below the photosensitive drum 101. Sheets112 are stored in a sheet cassette 109 in front of (i.e., on the rightside in FIG. 6) of the photosensitive drum 101, but sheet feed can alsobe implemented by hand feeding. A sheet feed roller 110 is disposed atan end of the sheet cassette 109 and feeds each sheet 112 in the sheetcassette 109 into the conveyance path.

The sheet 112 onto which the toner image not yet fixed was transferredas described above, is further transferred to a fixing unit locatedbehind the photosensitive drum 101 (i.e., on the left side in FIG. 6).The fixing unit is composed of a fixing roller 113 having a fixingheater (not illustrated) inside and a pressing roller 114 disposed inpress contact with the fixing roller 113 and heats while pressing thesheet 112 thus conveyed from the transfer part, in the nip part betweenthe fixing roller 113 and the pressing roller 114 to fix the unfixedtoner image on the sheet 112. Sheet discharge rollers 116 are disposedfurther behind the fixing roller 113 to discharge the fixed sheet 112 tothe outside of the image-forming apparatus.

Although not illustrated in FIG. 6, the print controller 111 alsoperforms control of each section in the image-forming apparatus,including the motor 115, and control of the polygon motor etc. in theoptical scanning unit described above, in addition to the conversion ofdata described above.

When the elements are set as described above according to the presentinvention, the optical scanning apparatus and the image-formingapparatus using it can be accomplished in the structure capable ofalways providing good images while controlling variation of the spotsize of the beam to a small value on the surface to be scanned, bydecreasing the focus movement amount even on the occasion ofsimultaneous occurrence of the change in the oscillation wavelength ofthe light from the light source means due to dispersion of theoscillation wavelength of the light from the light source means and/ordue to the ambient temperature change and the refractive index change ofthe materials of the optical systems due to the ambient temperaturechange.

In addition, according to the present invention, as described above, agood balance can be made between the focus movement due to the ambienttemperature change of apparatus and the focus movement due to theinitial wavelength change of the light from the light source means, byproviding the diffraction optical element in at least one optical systemout of the first imaging optical system placed between the light sourcemeans and the deflecting means and the second imaging optical systemplaced between the deflecting means and the surface to be scanned, andproperly setting the power of the diffraction optical element.

The need for adjustment of the cylindrical lens in the optical-axisdirection is eliminated by the balance between the two focus movementamounts, thereby accomplishing the advantages in terms of cost,including the decrease of assembling time, decrease of assembling tools,and so on.

What is claimed is:
 1. An optical scanning apparatus, comprising: lightsource means for emitting light; a first imaging optical system forconverging emitted light from said light source means; deflecting meansfor deflecting converged light from said first imaging optical system; asecond imaging optical system for scanning a surface to be scanned withdeflected light from said deflecting means; at least one refractionoptical element in each of said first imaging optical system and saidsecond imaging optical system; and at least one diffraction opticalelement in said first imaging optical system or in said second imagingoptical system, wherein a power of said diffraction optical element isin a subscanning direction and is set to a third power between a firstpower and a second power, wherein the first power is a power of said atleast one diffraction optical element when a focus movement in thesubscanning direction with respect to the surface to be scanned, causedby said at least one refraction optical element with a change of aninitial oscillation wavelength of the emitted light, can be canceled bya power change of said at least one diffraction optical element, andwherein the second power is a power of said at least one diffractionoptical element when a focus movement in the subscanning direction withrespect to the surface to be scanned, caused by said at least onerefraction optical element with a change of ambient temperature, can becanceled by a power change of said at least one diffraction opticalelement.
 2. The optical scanning apparatus according to claim 1, whereinsaid at least one diffraction optical element is provided in said firstimaging optical system.
 3. The optical scanning apparatus according toclaim 2, wherein said at least one diffraction optical element isprovided on a surface in said first imaging optical system closest tosaid deflecting means.
 4. The optical scanning apparatus according toclaim 3, wherein said first imaging optical system comprises a lenshaving a power in the subscanning direction and said at least onediffraction optical element is provided on a surface of said lens havinga power in the subscanning direction.
 5. The optical scanning apparatusaccording to claim 4, wherein fcl≦500/αs, wherein αs is a longitudinalmagnification of said second imaging optical system in the subscanningdirection, and wherein fcl (mm) is a focal length of said lens having apower in the subscanning direction.
 6. The optical scanning apparatusaccording to claim 5, wherein said lens having a power in thesubscanning direction includes no position adjusting means for adjustinga position of an optical-axis direction.
 7. The optical scanningapparatus according to claim 6, further comprising a third imagingoptical system for converging the deflected light and guiding convergedlight from said third imaging optical system into said light deflectingmeans, wherein said third imaging optical system includes an imaginglens having a power at least in the main scanning direction, and saidlens having a power in the subscanning direction and said imaging lensare integrally formed.
 8. The optical scanning apparatus according toclaim 5, wherein said lens having a power in the subscanning directionis formed from a plastic material.
 9. The optical scanning apparatusaccording to claim 1, wherein: |dΔS _(—) T|≧|dΔS_λ| if |dΔST _(—)λ|≧|dΔSλ _(—) T|, or |dΔS _(—) T|≦|dΔS_λ| if |dΔST _(—) λ|<|dΔSλ _(—)T|; and wherein dΔSλ_T is a focus movement amount with an ambienttemperature change when the power of said at least one diffractionoptical element is the first power; dΔST_λ is a focus movement amountwith a change of an initial oscillation wavelength of said light sourcemeans when the power of said at least one diffraction optical element isthe second power; dΔS_T is a focus movement amount with the ambienttemperature change; and dΔS_λ is a focus movement amount with the changeof the initial oscillation wavelength of said light source means whenthe power of said at least one diffraction optical element is the thirdpower.
 10. The optical scanning apparatus according to claim 1, whereinsaid at least one refraction optical element and said at least onediffraction optical element are disposed so that a focus movement amountwith a change of 1 nm in the initial oscillation wavelength is not morethan 0.3 mm.
 11. An optical scanning apparatus, comprising: light sourcemeans for emitting light; a first imaging optical system for convergingemitted light from said light source means; deflecting means fordeflecting converged light from said first imaging optical system; asecond imaging optical system for scanning a surface to be scanned, withdeflected light from said deflecting means; and at least one refractionoptical element in each of said first imaging optical system and saidsecond imaging optical system; and at least one diffraction opticalelement in said first imaging optical system or in said second imagingoptical system, wherein a power of said diffraction optical element is apower in a subscanning direction and is set to a third power between afirst power and a second power, wherein the first power is a power thatsaid diffraction optical element has when a focus movement in thesubscanning direction with respect to the surface to be scanned, causedby said at least one refraction optical element with a change of aninitial oscillation wavelength of the emitted light, can be canceled bya power change of said diffraction optical element; wherein the secondpower is a power that said diffraction optical element has when a focusmovement in the subscanning direction with respect to the surface to bescanned, caused by said at least one refraction optical element with achange of ambient temperature, can be canceled by a power change of saiddiffraction optical element, and wherein said first imaging opticalsystem includes a lens, which i) is formed from a plastic material, ii)has a power in the sub-scanning direction, and iii) satisfies thefollowing relationship: fcl≦500/αs, wherein fcl is a focal length ofsaid lens, and αs is a longitudinal magnification in the sub-scanningdirection of said second imaging optical system.
 12. The opticalscanning apparatus according to claim 11, wherein said at least onediffraction optical element is provided in said first imaging opticalsystem.
 13. The optical scanning apparatus according to claim 11,wherein: |dΔS _(—) T|≧|dΔS_λ| if |dΔST _(—) λ|≧|dΔSλ _(—) T|, or |dΔS_(—) T|≦|dΔS_λ| if |dΔST _(—) λ|<|dΔSλ _(—) T|; and dΔSλ_T is a focusmovement amount with an ambient temperature change when the power ofsaid at least one diffraction optical element is the first power; dΔST_λis a focus movement amount with a change of the initial oscillationwavelength of said light source means when the power of said at leastone diffraction optical element is the second power; dΔS_T is a focusmovement amount with the ambient temperature change; and dΔS_λ is afocus movement amount with the change of the initial oscillationwavelength of the emitted light when the power of said at least onediffraction optical element is the third power.
 14. The optical scanningapparatus according to claim 11, wherein said at least one refractionoptical element and said at least one diffraction optical element aredisposed so that a focus movement amount with a change of 1 nm in aninitial oscillation wavelength is not more than 0.3 mm.
 15. Animage-forming apparatus comprising said optical scanning apparatus as inany one of claims 1, 2-7, 9-10 or 11, and further comprising: aphotosensitive body placed on the surface to be scanned; a developingunit for developing an electrostatic latent image formed on saidphotosensitive body with deflected light into a developed toner image; atransfer unit for transferring the developed toner image onto a transfermedium; and a fixing unit for fixing the transferred toner image ontothe transfer medium.
 16. The image-forming apparatus comprising saidoptical scanning apparatus as in any one of claims 1, 2-7, 9-10 or 11,and further comprising a printer controller for converting code data,supplied from an external device, into an image signal and supplying theimage signal to said optical scanning apparatus.
 17. The opticalscanning apparatus according to any one of claims 4 through 6 or 11,wherein said lens comprises a cylindrical lens.